The present disclosure relates to a gas turbine engine and, more particularly, to a blade tip clearance control system therefor.
Gas turbine engines, such as those that power modern commercial and military aircraft, generally include a compressor to pressurize an airflow, a combustor to burn a hydrocarbon fuel in the presence of the pressurized air, and a turbine to extract energy from the resultant combustion gases. The compressor and turbine sections include rotatable blade and stationary vane arrays. Within an engine case structure, the radial outermost tips of each blade array are positioned in close proximity to a shroud assembly. Blade Outer Air Seals (BOAS) supported by the shroud assembly are located adjacent to the blade tips such that a radial tip clearance is defined therebetween.
When in operation, the thermal environment in the engine varies and may cause thermal expansion and contraction such that the radial tip clearance varies. The radial tip clearance may be influenced by mechanical loading, e.g., radial expansion of the blades and/or their supporting disks due to speed-dependent centrifugal loading and relative thermal expansion, e.g., of the blades/disks on the one hand and the non-rotating structure on the other. The radial tip clearance is typically designed so that the blade tips do not rub against the BOAS under high power operations when the blade disk and blades expand as a result of thermal expansion and centrifugal loads. When engine power is reduced, the radial tip clearance increases. The leakage of core air between the blade tips and the BOAS may have a negative effect on engine performance/efficiency, fuel burn, and component life.
To facilitate engine performance, at least some engines include a blade tip clearance control system to maintain a close radial tip clearance. To provide active control, some systems form the non-rotating structure with a circumferential array of BOAS mounted for controlled radial movement, e.g., via actuators such as electric motors or pneumatic actuators. An aircraft or engine control system may control the movement to maintain a desired tip clearance between the inner diameter faces of the BOAS and the blade tips. Additionally, various proposed systems have involved tailoring the physical geometry and material properties of the BOAS support structure to tailor the thermal expansion and provide a desired clearance when conditions change. Such thermal systems may be passive. Alternatively, such thermal systems may involve an element of active control such as selective cooling of cooling air to the support structure.